Wet flue gas denitrification is a technique employed to cleanse flue gas by washing, absorbing, and eliminating nitrogen. The majority of NOx present in the combustion flue gas consists of NO, which is not easily soluble in water. In the wet flue gas denitrification technology, the commonly used methods are acid absorption, alkali absorption, complex absorption and ozone denitrification. By using these method, the resulting compounds can be adsorbed or absorbed，and over 95% of the NOx can be removed. The following is an introduction of the above mentioned methods.
Acid Absorption Method
Acid absorption denitrification involves the absorption of NOx from flue gas using acidic substances like nitric acid, as the solubility of NOx in acid is significantly higher compared to water. Concentrated sulfuric acid is capable of effectively absorbing NOx. By leveraging this characteristic, both NO and NO2 can be absorbed into concentrated sulfuric acid, resulting in the production of nitrous sulfuric acid (NOHSO4), which can be subsequently recovered.
The efficiency of denitrification using the acid absorption method is influenced by various factors, including absorption temperature and pressure. However, this technique faces certain technical challenges, such as high energy consumption and a substantial need for acid circulation during the absorption process. These issues pose significant considerations in the implementation of the acid absorption method for denitrification.
Alkali Absorption Method
Alkali absorption denitrification involves employing alkaline solutions like NaOH, KOH, or NH3 · H2O as absorbents. The process of alkali absorption denitrification is relatively straightforward and allows for the recovery of denitrification by-products such as nitrites and nitrates. However, this method also has drawbacks, including lower denitrification efficiency and limitations in treating higher concentrations of NOx in the flue gas.
Complex Absorption Method
The predominant form of NOx in flue gas is NO, which is essentially insoluble in water and cannot be effectively captured in a liquid medium. To address this challenge, the wet complexation absorption method utilizes metal chelates like Fe(E)·EDTA, Fe(NTA), Fe(II)-EDTA, Fe(II)-EDTA-Na2SO3, and FeSO4. These chelates facilitate the rapid reaction and complex formation with dissolved NOx, particularly NO. Upon heating, the complex releases NO, thus concentrating it for subsequent recovery, further reduction, or oxidation treatment. This principle underlies the effectiveness of the wet complexation absorption method.
The complex absorption method demonstrates a high efficiency in removing NOx. However, it poses challenges in terms of chelate recycling. During the reaction, chelates are lost, leading to the deactivation of the absorbing liquid. Consequently, regeneration becomes difficult, resulting in low utilization and complicating the treatment of the waste liquid. These factors contribute to high operating costs associated with the complex absorption method.
Ozone is a high energy form of oxygen, with extremely strong oxidation. Ozone can effectively remove nitrogen oxides, sulfur dioxide, chlorofluorocarbons, etc., and can also be used for sterilization, decontamination, bleaching and deodorization purposes. In the process of breaking down chemicals, ozone will eventually be reduced to oxygen and water, causing no secondary pollution.
Ozone denitrification is mainly to use the strong oxidation of ozone to oxidize insoluble low-priced nitrogen oxides to soluble high-priced nitrogen oxides. Then, the nitrogen oxide of the high state is absorbed in the scrubber to achieve the purpose of denitrification.
The utilization of ozone in the field of oxidation and denitrification offers several advantages. Ozone exhibits a high selectivity towards NOx and acts rapidly, with the oxidation of NO being completed within approximately 0.05 seconds. To enhance the absorption of the oxidized products, a spray absorption process is employed at the backend. Notably, the flue gas does not require heating for effective absorption of N2O5 by the alkaline solution, and ozone also aids in the oxidation and removal of heavy metal pollutants like mercury. Furthermore, the presence of high concentrations of dust or solid particles in the flue gas does not significantly impact the removal efficiency of NOx. The production of ozone can be flexibly adjusted based on the concentration of NOx in the flue gas, allowing for adaptable treatment.
The complex oxidation reaction process of O3 for NOx primarily involves the alteration of the valence state of nitrogen (N). The key reactions include the following:
Benefits of Ozone Flue Gas Treatment
- The NOx removal rates can be configured to range from 30% to over 95%.
- Efficiently addressing a broad spectrum of nitrogen oxides and harmful pollutants is achievable, even in conjunction with diverse Air Pollution Control (APC) equipment.
- This forward-looking solution has the capability to meet the ever-changing NOx regulations, ensuring its relevance in the future.
- The capital expenditure (CAPEX) is low when the inlet NOx level is moderate and existing Air Pollution Control (APC) equipment can be utilized.
- The costs associated with ozone consumption are transparent and directly correlate with the efficiency of NOx removal.
- There is no requirement to interfere with or modify existing combustion equipment, nor is there a need to reheat the flue gas to high temperatures.
- There is no necessity to store hazardous chemicals typically used in traditional abatement processes.